"The Roof of the World" gives two different locations for the "Roof of the World". Susan maintains the Himalayas as the "Roof", while Marco Polo clearly states that the Plain of Pamir is. Though unexplained by the episode or the novelisation, they're both right. The Pamir Mountains are said to be at the intersection of the Himalayan, Tian Shan, Karakoram, Kunlun and Hindu Kush ranges. Thus, in a sense, Susan is right, but the "Plain of Pamir" is a more precise location.